A frame grab from CBS News shows an undated photograph of the suspect, Ivan Lopez, in the Fort Hood shooting on April 2. / CBS News

by Yamiche Alcindor, USA TODAY

by Yamiche Alcindor, USA TODAY

The soldier behind Wednesday's deadly shooting at Fort Hood was being evaluated for post-traumatic stress disorder, and had seen no combat while deployed in Iraq three years ago.

Also, the shooter bought his gun from the same place the 2009 Fort Hood shooter got his weapon.

Army officials Thursday afternoon identified the killer as Spc. Ivan Lopez, 34, a Puerto Rican father of three who authorities say had no record of misbehavior. Wednesday's tragedy at the Texas Army base left four people dead, including the gunman, and 16 injured.

"We have very strong evidence that he had a medical history that indicated an unstable psychiatric or psychological condition," Lt. Gen. Mark Milley, head of the Army's III Corps at Fort Hood, said of Lopez. "There was no indication that he was targeting specific people."

Milley hinted at a motive for the shooting. "There may have been a verbal altercation with another soldier or soldiers," he said. "There is a strong possibility that that immediately preceded the shooting."

Lopez purchased his gun on March 1.

Last year, Army psychiatrist Nidal Hasan was convicted and sentenced to death in the Nov. 5, 2009, attack at Fort Hood on his fellow soldiers as they waited inside a crowded building on the base. Thirteen died and more than 30 were wounded, and it remains the deadliest attack on a domestic military installation in U.S. history.

The Bureau of Alcohol, Tobacco, Firearms and Explosives on Wednesday night traced the gun used in Wednesday's attack to a local gun shop, said a federal law enforcement official not authorized to comment publicly. The official confirmed that the gun had been purchased at Guns Galore, the same shop that sold a weapon to Hasan.

Lopez enlisted in the Army in June 2008 and served four months in Iraq as a truck driver.

"His records show no wounds, no involvement - direct involvement - in combat," said Army Secretary John McHugh, the U.S. Army's top civilian official. "As Gen. Milley said, no record of Purple Heart or any injury that might lead us to further investigate a battle-related TBI (traumatic brain injury) or such."

Milley said Lopez had "self-diagnosed" a traumatic brain injury. "He was not wounded in action," Milley said.

On Thursday, McHugh said the suspected shooter had two deployments, including the one in Iraq. Lopez enlisted as an infantryman and later switched his specialty to truck driver.

Lopez, who was on a variety of prescribed drugs including Ambien, had not yet been diagnosed for post-traumatic stress disorder. But he was also undergoing treatment for depression, anxiety, sleep disturbance and a variety of other issues, McHugh said.

"He was seen just last month by a psychiatrist," McHugh said Thursday. "He was fully examined. And as of this morning, we had no indication on the record of that examination that there was any sign of likely violence, either to himself or to others. No suicidal ideation."

Out of respect for Lopez's family and the integrity of the investigation, Milley said he would not release any more details about the soldier's medical status. He did add that it was too early to tell if Lopez received adequate mental health treatment.

Officials had planned to continue to monitor and treat Lopez as deemed appropriate.

Authorities combing through the soldier's background have not found any evidence that he was involved in extremist organizations.

The motive for the shooting remains a mystery. The National Counterterrorism Center said that the attack is not linked to terrorism.

"He had a clean record in terms of his behavioral," McHugh said of Lopez. "No outstanding bad marks for any kinds of major misbehaviors that we are yet aware of."

On Wednesday, Lopez allegedly used a .45-caliber Smith & Wesson handgun to carry out the attacks. Milley said the shooter walked into a building on the post and opened fire, got into a car, fired more shots and then went to another building shooting before he was engaged by responding military police.

All those wounded and killed were military personnel.

Lopez joined the Army National Guard in Puerto Rico in 1999. Later, he joined the regular Army and served one year on the Sinai Peninsula in Egypt. He was assigned to the 1st Armored Division at Fort Bliss and transferred to the 154th Transportation Company in February at Fort Hood.

Lopez was assigned to the 13th Sustainment Command (Expeditionary). It's unclear when or where he was diagnosed with mental health issues.

Melissa Earle, an associate dean at the Touro College Graduate School of Social Work in New York City, cautions against assuming the suspect had post-traumatic stress disorder, which doesn't necessarily increase the risk of violence. Traumatic brain injury, which emerged as a major concern during the wars in Iraq and Afghanistan, is more closely tied to lack of impulse control than PTSD, she said.

Stereotyping those with PTSD as violent "could be a big setback to tens of thousands of U.S. vets with PTSD who are struggling for regular lives," Earle said.

Lopez lived off post and had recently purchased the gun used in the shooting. His wife was questioned Wednesday night. The couple are both natives of Puerto Rico.

Lopez had not been assigned to one of the Army Wounded Transition Units, which are set up to care for wounded, injured or ill soldiers. Those assigned to these units have case managers who help them track appointments and manage their medical treatments.

A commander who knows of a soldier with mental health issues living off post may require that service member to move to quarters on the post, said Army Col. Steve Warren, a Pentagon spokesman. Doing so would give the commander additional authority to deal with his or her issues, including the registering of weapons.

Pentagon regulations require troops who live off base to register weapons if they intend to bring them onto the installations, Warren said. Those weapons cannot be concealed and base security personnel conduct random checks to ensure compliance, he said.

"We try to do everything we can to encourage soldiers to register their personal weapons, even when they live off post," McHugh said. "We are not legally able to compel them to register weapons when they reside off post."

Soldiers who live on posts in base housing may also keep registered firearms. Soldiers in barracks must keep them in a locked arms room, Warren said.

The 2013 National Defense Authorization Act requires military mental health professionals to ask troops if they own a weapon or plan to buy one. It's not yet clear if Lopez was asked about his firearms.

Mental health professionals in the military, along with commanders, have a duty to warn and protect others from potentially dangerous troops, Warren said.

However, Warren acknowledged that on a post like Fort Hood, with 40,000 soldiers, checking every vehicle is not practical.

At the end of the rampage, a female soldier encountered the alleged shooter in a parking lot, Milley said.

Dressed in combat fatigues, Lopez reached for his weapon from under his jacket. The female soldier then pulled out her gun and "engaged" from about 20 feet away. Lopez then put the gun to his head and fired.